1. Field of the Invention
The present invention relates to a superconductive/normal conductive transition-type current-limiting element and current-limiting device that employs an oxide superconductive body.
2. Description of the Related Art
Fault current (short-circuit current) accidents in power circuits produce very large fault current flows. Fault currents are cut off by circuit-breakers, but even a breaker allows flow of fault current for a few tens of milliseconds, which generates a large electromagnetic force and massive Joule heat, causing the power devices and electric circuits to undergo considerable mechanical and thermal damage. A demand exists for development of a trouble-handling current-limiting device that can inhibit fault current in the event of such accidents, and thus support the function of circuit-breakers. Such a current-limiting device would have a major effect through stabilization of various power distribution systems, and with the ever-increasing complexity of modern-day systems, the rapid realization of such a current-limiting device is highly anticipated.
Numerous types of current-limiting devices have been proposed, but none of them are highly practical as of this writing. Those in relatively wide use include current-limiting reactors, permanent fuses, current-limiting cables and electric arc current-limiting devices, but these also have such problems as high electrical resistance and thus high heat generation when carrying load current, or a slow response resulting in poor current-limiting performance.
Some of the qualities required for current-limiting devices include low electrical resistance and minimal heat generation when carrying normal load current, as well as a fast response with high electrical resistance in the event of short-circuit accidents. From this standpoint it is thought that current-limiting devices employing superconductors are ideal. Superconductive current-limiting devices that have been proposed include superconductive/normal conductive transition types, reactor types, rectifier types and coil types. Among these, superconductive/normal conductive transition-type current-limiting devices most directly utilize the superconduction/normal conduction transition properties of superconductors. When carrying a load current, the current flowing to the superconductor is never greater than the critical current, and therefore the resistance of the superconductor is extremely low. In the event of a short-circuit accident, however, the current flowing to the superconductor exceeds the critical current, and the heat generated thereby causes transition of the superconductor from a superconductive state to a normal conductive state, thus generating electrical resistance. This resistance limits the fault current. The superconductive/normal conductive transition-type current-limiting device is characterized by having a simpler construction and smaller size than other types of superconductive current-limiting devices.
The superconductive/normal conductive transition-type current-limiting element in which a load current flows directly in a superconductor as above is also called a xe2x80x9cresistance-type current-limiting elementxe2x80x9d and is disclosed, for example, in U.S. Pat. No. 5,235,309 to Preisler et al. There is also a so-called xe2x80x9cmagnetic shield-type current-limiting elementxe2x80x9d in which a load current does not flow directly in a superconductor and, in the event of a short circuit, the magnetic shield effect of a superconductor lowers thereby rapidly increasing the inductance of a magnetic circuit, which is disclosed, for example, in U.S. Pat. No. 5,355,275 to Goodier et al.
The superconductive current-limiting device described above may employ a metal-based superconductor or an oxide-based superconductor. Metal-based superconductors have been associated with problems such as low electrical resistance in a normal conductive state and requirements for larger-sized apparatuses and use near liquid helium temperatures that result in higher operating costs, while the apparatuses are also larger from the standpoint of thermal insulation. Demand is therefore high for the development of current-limiting devices employing oxide based superconductors with high electrical resistance in the normal conductive state, where the superconductive state can be maintained with cheaper liquid nitrogen.
Superconductive current-limiting devices employing oxide superconductive materials in the published literature include a type wherein a thin-film current-limiting element is formed on a substrate (Japanese Unexamined Patent Publication No. 2-281766) and a type using a bulk sintered body (Minutes of the Power and Energy Group Meeting at the 1995 Electrical Convention, p. 697). The type of device employing a thin-film has a high critical current density but a low cross-sectional area, and therefore does not attain a current value at the level used in actual low-voltage systems. On the other hand, sintered bodies have a large cross-sectional area but a low critical current density, and therefore attain a similar level. Thus, the development of a current-limiting device employing an oxide superconductive material with a current capacity that can withstand use in actual systems is an important issue.
Specifically, because oxide superconductive materials are used at relatively high temperatures, they have been associated with the problem of susceptibility to blowout with local temperature increase, as a consequence of their higher specific heat in the temperature range of their use compared to metal-based superconductive wiring used at near 4.2 K, as well as their low thermal conductivity which hampers propagation at areas that have undergone transition to normal conduction. This susceptibility to blowout can also be attributed to the fact that oxide superconductors are poorly suitable for achieving fine gauges and uniformity, compared to metal-based wiring materials.
The present invention overcomes the problems described above by providing a current-limiting device employing a bulk superconductor with a large current capacity, which device has a fast response and generates uniform quenching without blowout.
It is the gist of the present invention that in a current-limiting device including a superconductive/normal conductive transition-type current-limiting element employing an oxide superconductor wherein RE2BaCuO5 (where RE is at least one element selected from among Y, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb and Lu) is finely dispersed in an REBa2Cu3O7xe2x88x92x, phase, it is extremely effective to provide a mechanism which promotes or generates quenching in addition to the conduction current, in order to minimize local quenching due to non-uniformity of the superconductive properties, to accomplish transition of the current-limiting element from a superconductive state to a normal conductive state, and to achieve a higher response speed. According to the invention, xe2x80x9cquenchingxe2x80x9d refers to a sudden transition from superconduction to normal conduction.